Gas and electric-arc welding produces an extremely bright light that is rich in ultraviolet and infrared rays. Frequent or prolonged exposure to such light can blind a person. For this reason welders usually wear protective helmets or goggles provided with filters designed to stop harmful rays.
The principal difficulty with such a light-filtering device is that visible light is also greatly attenuated. Thus it is customary for the welder to strike the arc or ignite the gas flame with the goggles or helmet tilted up so that he can see what he is doing, and to put this protective gear in place only when the light generated by the welding operation is sufficient to allow him to view the workpiece. This short but frequent exposure to the unfiltered light from the arc or flame is in the long run very harmful.
It has already been proposed (see, for example, U.S. Pat. No. 3,245,315) to use an electro-optical shutter in a pair of spectacles designed to protect the eyes of the user against excessively brilliant flashes of light. A similar shutter or variable-density filter, utilizing the birefringence of a ferro-electric ceramic plate, is described in U.S. Pat. No. 3,737,211. More recently, so-called liquid crystals have been envisaged for the same purpose; see U.S. Pat. No. 3,873,804.
A shutter using a birefringent plate or a nematic-phase liquid as its electro-optical switching element generally comprises a polarizer and an analyzer between which that element is sandwiched. Thus, if the analyzer and the polarizer are relatively oriented in such a manner that incident unpolarized light is able to pass through the stack with only moderate attenuation, the application of a switching voltage to the electro-optical element under the control of a photocell responsive to a bright flash may reduce the light transmissivity of the shutter assembly to almost zero.
As disclosed in my copending application and patent identified above, such a shutter (when used as an eyeshield for a welder's headpiece) can be placed in its condition of maximum attenuation by a switchover to either an energized or a de-energized state of its electro-optical element. Although with certain liquid crystals the transition from high to low light transmissivity may take slightly longer when achieved by the removal of voltage rather than by the generation of an electric field, dimming through de-energization has the advantage that the eyes of a welder will not be suddenly exposed to the glare of an arc in the event of a failing current supply.
As described in the above-cited U.S. Pat. No. 3,873,804, a nematic-phase liquid crystal normally imparts a 90.degree. rotation to plane-polarized light impinging with a predetermined orientation upon one of its cell walls. When an electric field of a certain minimum strength is applied across these walls, the molecules of the liquid are reoriented (from a homogeneous state paralleling the cell walls to a homeotropic state perpendicular thereto) so that such rotation will no longer occur. With crossed planes of polarization of the polarizer and the analyzer respectively adjoining these cell walls, the light transmissivity of the energized shutter drops to a small fraction of its previous value.
Light traversing a liquid crystal in the homogeneous state of its nematic-phase liquid experiences considerable scattering so that the crystal appears cloudy. Thus, the user looking through such a shutter in its high-transmissivity or "open" condition does not have a very clear view of a workpiece to be welded, for example. When the shutter is dimmed by a switchover to the homeotropic state, according to conventional practice, the scattering effect is greatly diminished so that a welder will be able to see the flame-illuminated workpiece even through the darkened cell assembly.
With a reverse arrangement, i.e. with the polarizer and the analyzer effective in parallel planes of polarization so that light transmission will be a maximum in the presence of an electric field, the cloudiness in the "open" state of the shutter disappears but the light-scattering effect in its "closed" state further reduces the visibility of the workpiece during the welding operation to an extent making it difficult for the operator to see what he is doing.